1. Field of the Invention
The present invention relates to a power circuit for driving a liquid crystal display panel, and more particularly to a circuit for quickly lowering the electric potential inside the driving power circuit in a very short period of time after having turned off the power circuit for driving the liquid crystal display panel.
2. Related Art
As well known, the liquid crystal display panel is a device which is generally made up of two electrode plates provided with a plurality of electrodes and a liquid crystal as is put between these two electrode plates, and displays an image on the display panel when a predetermined voltage is applied to the liquid crystal through respective those electrodes. In this specification, the electrode provided on the above two electrode plates and led in the lateral direction is called a common electrode (referred to as xe2x80x98COMxe2x80x99 hereinafter) while the electrode provided on the above two electrode plates but led in the longitudinal direction is called a segment electrode (referred to as xe2x80x98SEGxe2x80x99 hereinafter). When the potential difference between the COM and the SEG is equal to or larger than a predetermined value, the liquid crystal positioned at the intersection of the COM and the SEG is turned on while the above liquid crystal is turned off (non-lit state) when the above potential difference is smaller than the predetermined value.
FIG. 7 is a diagram showing an example of a wave form describing the relation between the potential difference between the COM and the SEG, and the lights-on/lights-out of the liquid crystal. In this example, the voltage level (electric potential) of the COM or the SEG is in a relation of V1 greater than V2 greater than V3 greater than V4 greater than V5 greater than VSS (VSS: the ground potential GND), and the liquid crystal at the intersection of the COM and the SEG is set to be in the lights-on state if the potential difference between the COM and the SEG is equal to or higher than |V1| while the above liquid crystal is set to be in the lights-out state if the potential difference between the COM and the SEG is lower than |V1|.
Up to now, the driving power circuit for the liquid crystal display panel has employed a power down short circuit in order to shorten the fall time of the electric potential inside the driving power circuit after turning off the power source of the liquid crystal display panel. FIG. 8 is a diagram for explaining the change of respective power source potentials V1xcx9cV5 within the driving power circuit when operating the power down short circuit, that is, the change of respective power source potentials V1xcx9cV5 from the time of starting the operation of the power down short circuit after turning off the power source of the liquid crystal display panel to the time of ending the operation of the power down short circuit.
The power down short circuit as described above will now be explained with reference to FIG. 9. In the power down short circuit 50 as shown in FIG. 9, in order to detect if the power source of the liquid crystal display panel has been certainly turned off, in other words, that the level of the VDD power source has surely dropped down, a certain arbitrary level difference is generated between nodes B and C by means of condensers C1 and C2 for generating the level difference and diodes D1 and D2 for generating the level difference. A resistance R1 provided on the VDD power source line, resistances R2xcx9cR6 provided on each of power source lines, and a resistance R7 provided on the power source line of a level comparator 51 are noise absorption resistances for absorbing noises which might be included in the corresponding power source lines. A logical value of a node A is inverted by means of the level comparator 51, based on the level difference between the nodes B and C.
That is, when detecting if the level of the VDD power source has dropped down (i.e. when the power source of the liquid crystal display panel is turned off), the level of the node A is inverted from the L-level to the H-level. With this inversion of the node A to the H-level, a plurality of N-channel MOS transistors M1xcx9cM5 are turned on, thereby short-circuiting the power lines VL1xcx9cVL5 provided corresponding to voltage levels V1xcx9cV5 to the ground.
By the way, it is a general way that some external condensers are fitted to the driving power circuit of the liquid crystal display panel for stabilization of the voltage level. Furthermore, in case of making use of the power source lines VL1xcx9cVL5 of the above-mentioned driving power circuit for instance, it is needed to keep the relative level height relation existing among the voltage levels of the power source lines unchanged, taking account of the relation of the power sources for the lights-on and the lights-out. Accordingly, when applying the driving power circuit to the liquid crystal display device having the characteristic as shown in FIG. 7, it is needed to keep the relative level height relation existing among voltage levels, that is, V1 greater than V2 greater than V3 greater than V4 greater than V5, unchanged.
As described in the above, the power down short circuit starts its operation as soon as the power circuit for driving the liquid crystal display panel is turned off, thereby short-circuiting the power source lines VL1xcx9cVL5 to the ground. However, the voltage levels V1xcx9cV5 corresponding to respective power source lines VL1xcx9cVL5 can not always fall within an adequately short period of time due to the influence of the external condenser, and this causes such a problem that a certain kind of afterglow comes out on the liquid crystal panel. Furthermore, the operation of the power down short circuit can short-circuit the power source lines VL1xcx9cVL5 to the ground, but the falling speed of respective voltage levels can not be always uniform but is apt to become unbalanced. This also causes the problem that a certain kind of afterglow comes out on the liquid crystal display panel.
The present invention has been made in view of the above-mentioned problems as have been experienced so far in connection with the prior art power circuit for driving the liquid crystal display panel. Accordingly, the object of the invention is to provide a novel and improved power circuit for driving the liquid crystal display panel, which is able to prevent any afterglow from coming out on the liquid crystal display panel when the power circuit for driving the liquid crystal display panel is turned off.
In order to solve the problems as described above, according to the first aspect of the invention, there is provided a power circuit for driving a liquid crystal display panel, which includes a voltage generator for generating a plurality of voltage levels for driving a liquid crystal display panel; a plurality of power source lines provided corresponding to the plurality of voltage levels; a detector for detecting if the liquid crystal display panel has been turned off; a grounding circuit for grounding the plurality of power source lines in response to the detection result obtained by the detector; and a circuit for shorting predetermined power source lines each other in response to the detection result obtained by the detector, wherein the short circuit shorts adjacent power source lines from among the plurality of power source lines each other.
According to the driving power circuit having such a structure as described above, when the detector detects if the liquid crystal display panel has been turned off, a plurality of power source lines are grounded to reduce each electric potential thereof and, at the same time, adjacent power source lines can be short-circuited each other. Consequently, it becomes possible to shorten the fall time of the electric potential of each power source lines. In this specification, the expression xe2x80x9cadjacent power source linesxe2x80x9d indicate such two power source lines from among a plurality of power source lines that are positioned side by side when arranging all of them in the relative level height order of the electric potential of the power source lines.
As described above, the liquid crystal element of the liquid crystal display panel is lighted when the electric potential difference between the COM and the SEG becomes equal to or higher than a predetermined electric potential (|V1|, for instance). However, after turning off the power source of the liquid crystal display panel, if there still exist some liquid crystal elements having an electric potential near the above predetermined electric potential (V2xe2x88x92VSS=|V2|, for instance) and the electric potential of the power source falls slowly, it happens that those liquid elements are lighted by giving a certain potential difference to them for a certain period of time. That is, this causes an afterglow phenomenon. According to the invention, however, the electric potential of each power source line can be reduced in a shorter fall time, so that the afterglow can be prevented from coming out on the liquid crystal display panel.
Furthermore, according to the second aspect of the invention, there is provided a power circuit for driving a liquid crystal display panel, which includes a voltage generator for generating a plurality of voltage levels for driving a liquid crystal display panel; a plurality of power source lines provided corresponding to the plurality of voltage levels; a detector for detecting if the liquid crystal display panel has been turned off; a grounding circuit for grounding the plurality of power source lines in response to the detection result obtained by the detector; and a circuit for shorting predetermined power source lines in response to the detection result obtained by the detector, wherein the short circuit shorts adjacent power source lines from among the plurality of power source lines without inverting the relative level height order of the electric potential of the plurality of power source lines.
According to the driving power circuit having such a structure as described above, the power source lines are short-circuited without inverting the relative level height order of the electric potential of the power source line, so that the electric potential of each power source line can be reduced in a shorter fall time. For instance, it is possible to always keep the relation of V1 greater than V2 greater than V3 greater than V4 greater than V5( greater than VSS) unchanged. As will be described later, in order to prevent the afterglow from coming out on the liquid crystal display panel, it is effective to reduce each electric potential level of the power source lines, always keeping the relation of V1 greater than V2 greater than V3 greater than V4 greater than V5( greater than VSS) unchanged.
Still further, according to the third aspect of the invention, there is provided a driving power circuit for a liquid crystal display panel, which includes a voltage generator for generating a plurality of voltage levels for driving a liquid crystal display panel; a plurality of power source lines provided corresponding to the plurality of voltage levels; a detector for detecting if the liquid crystal display panel has been turned off; a grounding circuit for grounding the plurality of power source lines in response to the detection result obtained by the detector; and a circuit for shorting predetermined power source lines each other in response to the detection result obtained by the detector, wherein the short circuit shorts the power source lines provided in correspondence with the lights-out level of the COM electrodes of the liquid crystal display panel and also short-circuits the power source lines provided in correspondence with the lights-out level of the SEG electrodes of the liquid crystal display panel.
According to the driving power circuit having such a structure as described above, as the short circuit shorts the power source lines provided corresponding to the lights-out level of COM, it become possible to surely discharge the capacitance as charged up on the side of COM. As the short circuit also short-circuits the power source lines provided corresponding to the lights-out level of SEG, it become possible to surely discharge the capacitance as charged up on the side of SEG. Accordingly, the lights-on of the liquid crystal element caused by the longer fall time of the power source line potential is prevented, thus the afterglow being prevented from coming out on the liquid crystal display panel.
Still further, according to the fourth aspect of the invention, there is provided a power circuit for driving a liquid crystal display panel, which includes a voltage generator for generating a plurality of voltage levels for driving a liquid crystal display panel; a plurality of power source lines provided corresponding to the plurality of voltage levels; a detector for detecting if the liquid crystal display panel has been turned off; a grounding circuit for grounding the plurality of power source lines in response to the detection result obtained by the detector; and a circuit for shorting predetermined power source lines each other in response to the detection result obtained by the detector, wherein the short circuit shorts the power source line provided in correspondence with the maximum voltage level and each of the other power source lines each other.
According to the driving power circuit having such a structure as described above, as the short circuit shorts the power source line provided corresponding to the maximum electric potential level and each of the other power source lines each other, the electric potential of each power source line can be reduced in a shorter fall time and also can be held equal to or lower than the maximum electric potential level. For instance, it is possible to always hold the relations of V1xe2x89xa7V2, V1xe2x89xa7V3, V1xe2x89xa7V4 and V1xe2x89xa7V5. Therefore, it become possible to surely reduce each electric potential of the power source lines provided corresponding to the electric potential (V2xcx9cV5) lower than the V1, thus the afterglow being prevented from coming out on the liquid crystal display panel.